1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor laser device, and more particularly to a method of manufacturing a semiconductor laser device having a light shield plate.
2. Description of the Related Art
A laser which mainly emits bluish purple light can be manufactured using semiconductor material. Semiconductor laser devices can emit laser light approximately from ultraviolet rays of wavelength 360 nm to bluish green light of wavelength 490 nm, and lasers in blue and purple regions of wavelengths 400 through 450 nm are used in various fields. Semiconductor laser devices having a wavelength in the proximity of 405 nm are used as a light source of next-generation high-capacity optical storage media such as blue-ray disks and high-definition digital versatile disks (HD DVD). Semiconductor laser devices having a wavelength in the proximity of 450 nm can be used as blue light source of laser display systems. When semiconductor laser devices having wavelengths of 500 nm or higher are available, the semiconductor laser devices could also be used as a green light source of laser display systems. Also, blue-purple semiconductor laser devices could be used as a light source of high-resolution laser printers. Semiconductor laser devices having short wavelengths of 400 nm or lower in the ultraviolet ray region can be manufactured using nitride semiconductor materials and applied for a biological or medical use.
In nitride semiconductor laser devices, when Al composition in the n-clad layer, which is formed of AlGaN, is not sufficiently high or when the n-clad layer is not sufficiently thick, the optical confinement weakens, and thus light can be leaked from a lower surface of the n-clad layer.
In nitride semiconductor laser devices employing a sapphire substrate, light leaked from a lower surface of the n-clad layer exists in an n-contact layer between the sapphire substrate and n-clad layer, and a portion of the leaked light further leaks out of the laser device through a cross-sectional end of the substrate and the n-contact layer that is the end of the laser device from which the laser beam emerges. Also, in nitride semiconductor laser devices grown on a GaN substrate, light leaked from a lower surface of the n clad-layer exists inside the substrate, and a portion of the leaked light further leaks out of the laser device through a cross-sectional end of the substrate that is the end of the laser device from which the laser beam emerges. The leaked light interferes with a far-field pattern of the laser beam emerging from the semiconductor laser devices, illustrated by the formation of ripples in the graphs shown in FIGS. 1A and 1B.
The ripples in the far-field pattern can cause problems in applying the blue-purple semiconductor laser device to a system. For example, when using the blue-purple semiconductor laser devices as blue light source of laser displays, the ripples make display images uneven and thus deteriorate the quality of screen displays. Also, when used as a light source of optical storage media, the ripple shapes increase noise, and thus, errors in reading signals during information reproduction, which causes problems for reliability of the optical pickup.
To decrease the ripples in the far-field pattern, light leakage down from the n-clad layer should be blocked. To do that, the optical confinement needs improving by increasing the Al composition ratio in n-clad layer or thickening the thickness of n-clad layer. However, these methods are limited because compositions comprising excessive amounts of Al or excessively thick n-clad layers increase the probability of inducing cracks during growth for semiconductor laser devices. Light leakage also increases for longer wavelengths of light, and that could be a serious drawback with respect to applications such as a source for laser displays.
Japanese Laid-Open Patent No. 2005-101457 discloses a technique which stops light leakage through a substrate by depositing a light shield membrane on the cross-sectional end of the substrate that is on the end of the laser device through which a laser beam emerges. According to the Japanese Publication, a semiconductor laser device is attached to a jig which can screen a region where the light shield membrane should not be formed on the light emission face of semiconductor laser device, so that the light shield membrane can then be deposited on appropriate regions of the end of the substrate.